1. Field of the Invention
The invention relates to network communications and, more particularly, to a method for switching channels in a wireless communication network comprising at least one access point and a number of clients, where the communication network uses one of a plurality of channels as a secondary user for data communication in a predetermined frequency range. The invention also relates to an access point of a wireless communication network.
2. Description of the Related Art
The Industrial, Scientific and Medical (ISM) frequency band extends across the frequency range from 5.15 GHz to 5.725 GHz, is largely free of license payments and is globally available. For this reason, the ISM frequency band is used by users not only for wireless communication networks based, for example, on the WLAN (Wireless Local Area Network) standard or on Bluetooth, but also by radar systems at airports or on ships. As a result, there are rules to facilitate cooperative use of the ISM frequency band, because of such possible multiple uses in this frequency band.
The regulatory authorities have divided the users (i.e., the wireless communication networks and also radar applications) into primary and secondary users, which is equivalent to a prioritization. In Germany and many European countries, this subdivision applies particularly to the 5.25 GHz to 5.35 gigahertz (i.e., channel 52 to 64) and 5.47 to 5.725 GHz (i.e., channel 100 to 140) bands. In accordance with certain rules, a wireless local network (WLAN) forming a secondary user must withdraw into an outer band as soon as a radar application is detected by the user, because the application is basically a primary user. As a result, there is a necessity of the WLAN to search the channel used by the communication network for primary users. On detection of a primary user, the channel must be released by the WLAN and a switch must be made to another channel that is unused by a primary or secondary user. This capability is referred to as Dynamic Frequency Selection (DFS).
Here, the access point of the WLAN must periodically listen to the channel used by it to detect the primary user. This is referred to as a Channel Availability Check (CAC). As soon as a foreign sender is detected, the access point blocks this channel for use by its communication network for a specific time and initiates the channel switch to a free channel. In such cases, the new channel is selected at random and the selection is generally performed by the access point. A check must initially be performed on this new channel for a predetermined period of time, as a rule 60 seconds, to determine whether a primary user is present. This check is referred to as a Channel Observation Time (COT). If no foreign user is found, a channel switch to this free channel occurs. The switch is also performed by the clients of the wireless communication network. If another system is detected during the COT, the WLAN access point also blocks this channel for a specific time and checks the next channel for the predetermined time to detect the presence of other systems.
The interruption of data communication of the wireless communication network is not only a consequence of a forced channel switch, but also frequently occurs after 24 hours to perform an exhaustive check of all radio channels. Consequently, data communication in the wireless communication network is interrupted at least once a day. Some systems perform this check during operation, where an exhaustive check of the channels is only performed in the event of suspicion of other systems.
A problem associated with a channel switch is the handover time of a respective WLAN client once the access point has detected a primary user. As a rule, the channel switch is linked to a communication outage ranging in time over a number of beacon intervals, which is the result of the initiated search for a free channel. Even if the access point already knows about a free channel, the communication outage lasting for a number of beacon intervals is the eventual result. Typically, a beacon interval amounts to between 20 and 1000 ms. The reason for the communication outage is because although the access point transmits information in a beacon (i.e., a Channel Switch Announce (CSA)) immediately after a primary user has been detected, with which the clients are informed about the channel on which further communication is to occur, the access point itself does not however perform this channel switch immediately. This is because, the access point remains for a further number of beacon intervals on the old channel to ensure that all clients of the wireless communication network are actually switching channels and to send out the CSA element. Here, the clients are only allowed to re-communicate when they receive a beacon from the access point in which there is no CSA element on the new free channel.
It is basically of importance, especially in industrial use of a wireless communication network, for a defined quality of the communication network to be provided for data throughput and real-time capabilities. This quality is only provided, however, if the behavior of all points (i.e., access points and clients) of the communication network is able to be planned and predicted. Only in this way are reliable radio connections able to be realized.